1. Field of the Invention
The present invention relates to a pixel modulation apparatus and a method thereof, and more particularly to a pixel modulation apparatus capable of performing high-accuracy pulse width modulation for controlling the light emission of a laser on a pixel basis in an image forming apparatus which uses the laser to form an image, and a method of such pixel modulation.
2. Related Background Art
An image forming apparatus using a laser beam has been used as one of apparatus performing the control of the quantity of a laser beam by means of a pulse width modulation. As for such an image forming apparatus, first, a color image forming apparatus is simply described as an example thereof.
A color image forming process of such an color image forming apparatus generally uses four kinds of toners of yellow (Y), cyan (C), magenta (Mg) and black (Bk) severally. Such a color image forming process takes a time four times as long as the time necessary for an image forming process of a conventional image forming apparatus forming a monochrome image if no measures for shortening the time are taken. Because of this, an image forming process adopts use of four photosensitive drums for the respective four colors and use of a two-beam laser capable of writing two lines at the same time.
FIG. 1 is a schematic diagram of a conventional four-drum type image forming apparatus. In the apparatus, photosensitive drums 18a, 18b, 18c and 18d are disposed in a line. A different color is allotted to each of the photosensitive drums 18a-18d. The toner of each color is sequentially transferred to a photographic printing paper 26, and a color image is reproduced on the photographic printing paper 26. Each of the photosensitive drums 18a-18d is provided with an image writing portion shown in FIG. 2. The image writing portion forms an electrostatic latent image on the photosensitive drum by means of a laser beam. The operations of the image writing portion shown in FIG. 2 are described in the following.